1. Field of the Invention
This invention relates to a method an apparatus for providing a continuously adjustable phase shift to an electronic signal.
2. Description of the Prior Art
Many methods of continuously phase shifting an electronic signal are known. Of the known methods, those which are capable of phase shifting a signal 180 degrees or more utilize cascaded stages which are each adjusted, or otherwise utilize expensive variable inductive or capacitive components, such as tapped delay lines.
Other methods of continuously shifting phase of a signal have utilized a variable resistor (pot) such as shown in FIG. 1. The phase shift is accomplished by applying different phases of the signal to each end of the variable resistor, which allows a given phase between the two applied phases to be selected by in effect mixing a portion of each of the applied signals. Unfortunately, if the applied phases differ by 180 degrees the two components will exactly cancel thereby eliminating the output signal. More than 180 degree adjustment is not possible with this arrangement, as the application of fixed phases of more than 180 degrees will add vectorially giving a range of less than 180 degrees. The aforementioned problem can be eliminated by various switching arrangements, one of which is shown in FIG. 1, however this adds more expense, and requires that a switch be adjusted in addition to the pot.
A device called a goniometer solves all of the above problems quite nicely by providing a rotating coil inside of three or more stationary coils, the stationary coils being driven with different phases of the input signal. Thus, the rotating coil provides different output phases by being rotated to the appropriate position in proximity to the stationary coils. Unfortunately the cost of a goniometer is relatively high due to the coil construction, and the problem of coupling the output of the rotating coil member to the outside via slip rings or rotary transformer.
Other solutions to the problem include various electronic circuits such as phase locked loops, all pass filters and electronically variable delays, all of which are relatively complicated and expensive.